The mechanism of release of alpha subunit from dimeric alpha beta AMV DNA polymerase was elucidated in great detail. Functional differences between the RNase H activity of alpha and alpha beta forms were determined by immunological data. Antisera against alpha and alpha beta forms inhibit the DNA polymerase activity of both forms but only the RNase H activity of the enzyme form used as immunogen. Antibody specifically inhibitory for alpha beta RNase H activity can be absorbed by purified alpha, and antibody specifically inhibitory for alpha RNase H activity can be absorbed by purified alpha beta. These data indicate that antibodies against alpha and alpha beta forms of AMV reverse transcriptase recognize distinct antigenic determinants present on alpha and that the alpha and alpha beta forms differ in the steric relationship of these determinants to the RNase H active site. Active site directed reagent (pyridoxal 5'-phosphate) revealed that the deoxynucleoside triphosphate binding site is located on the alpha subunit. The alpha subunit possesses five reactive amino groups, one of which is essential for catalytic activity; the beta subunit has three reactive amino groups which are not involved in the deoxynucleoside binding site. BIBLIOGRAPHIC REFERENCES: Papas, T.S., Marciani, D.J., Samuel, K. and Chirikjian, J.G.: Mechanism of release of active alpha subunit for dimeric alpha beta avian myeloblastosis virus DNA polymerase. J. Virology 18: 904-910, 1976. Papas, T.S., Dahlberg, J.E. and Sonstegard, R.A.: Type C virus in lymphosarcoma in northern pike (Esox lucius). Nature 261: 506-508, 1976.